One of the things I have learned in helping friends to plan trips is to listen to their needs and wants and plan the trip for them...not the trip that *I* would want for them.
In this situation, like you,OP, *I* would want for them to have the lowest crowd experience possible, etc. But this is where I stop and think about what they love and what they would want to experience. It sounds like Star Wars would be a huge deal to her kids, and knowing that, I would recommend DHS even knowing it'll have a higher crowd level. I would be up front about the higher crowds, and advise her to get there as early as they can (up to them, obviously) or to stay as late as they can, and possible ways to mitigate the mid-day crowds (perhaps doing shows like Indiana Jones, which have high capacity, mid-day) - but I wouldn't recommend that they stay away from the park that the 2 boys would probably love the most right now.
Like a PP said...just because it isn't what we would choose, or how we would plan, doesn't mean the trip will be miserable. A friend of mine goes with her extended family every 4th of July. We're talking 3 generations of people...at least 20 people total. They have a great time each year. It may not be my cup of tea, but it works for them, and that's all that matters.